The role of the enzymes aldose reductase and aldehyde reductase in the onset and progression of complications associated with diabetes are being investigated. In the eye these complications include retinopathy, cataract, keratopathy, changes in pupil function, and iris and ciliary process structure changes. Under development are methods of pharmacological control of these enzymes to either delay or prevent the onset and progression of these complications. Events leading to the formation of several types of cataracts, as well as methods for controlling the onset of these cataracts through pharmacological intervention, are also being investigated.